The field of this invention is radar and more particularly, infrared imaging radar.
In conventional microwave radar systems, electromagnetic radiation from a power oscillator is directed by a transmit/receive switch, through an antenna onto a scene-to-be-imaged. The radiation reflected by targets within the scene is collected by an antenna and directed through the transmit/receive switch into a mixer where it is heterodyned with microwave radiation from a local oscillator. The heterodyned signal is then electronically processed to extract range, velocity and reflectivity information for the targets.
Conventional infrared radar systems also transmit electromagnetic radiation but rather from a transmitter laser typically, the laser radiation is directed by a transmit/receive switch, through a telescope and onto a target. The radiation reflected by the target is then collected by the telescope and directed through the transmit/receive switch and onto an optical detector. This detected radiation is heterodyned with radiation from a local oscillator laser. The heterodyned signal is then electronically processed to extract range, velocity and reflectivity information for the targets. The principle differences between the microwave and infrared radar systems are the wavelength of the electromagnetic radiation employed (microwave versus infrared), and the specific devices which perform the necessary radar functions (e.g. magnetrons versus lasers, antennas versus telescopes, and the like). In view of the wavelength difference, infrared systems generally offer higher resolution, while microwave systems generally offer better performance in bad weather.
Currently, microwave radar systems provide terrain following capability, permitting all-weather, day/night low level flight aircraft operation at extremely low altitudes and relatively high speeds. However, at low altitudes, for example, below 50 meters, many flight applications require a substantial obstacle avoidance capability in addition to terrain following. Since obstacle detection requires extremely high resolution, that is not practical with microwave radars.
Infrared laser airborne radar systems have recently been developed for use as bad-weather, day/night, obstacle avoidance systems on tactical aircraft involved in close air support missions. See for example Hull, R. J., Marcus, S. "A Tactical 10.6 um Imaging Radar", Proc. 1978 National Aerospace and Electronics Conf. (IEEE, Dayton, Ohio, May 1978). In such systems, there are two modes of operation. In a target acquisition (or obstacle detection) mode, a CO.sub.2 laser generates a continuous wave (CW) infrared beam. This beam is shaped into a fan beam and projected through a telescope and directed by a pointer scanner mirror assembly onto the ground in front of the aircraft. A combination of the aircraft's forward motion and a horizontal rocking motion of the pointer-scanner mirror assembly provides a line scan search of the area in front of the aircraft. The back-reflected radiation is collected by the telescope, and imaged onto a one-dimensional array of of heterodyne detectors together with the beam from a local oscillator laser. The outputs from the heterodyne detectors are then Doppler-analyzed to provide a moving target indication (MTI). When a moving target (or obstacle) is detected, the pointer-scanner mirror is adaptively pointed in the direction of the target, and the transmitter laser is switched to an obstacle avoidance mode. In this second mode, the system operates as a laser-aided forwarded looking infrared imaging (FLIR) system. The laser is repetitively-pulsed, and a two-dimensional image plane scanner is activated. This second mode of operation provides a high resolution point-by-point raster scanned CRT image of the target for identification purposes. Range information from the reflected pulse delays are then used in conjunction with the azimuth-elevation information from the image and information from the aircraft's inertial platform to provide obstacle avoidance control signals for the aircraft. This form of system is also suitable for fire control.
Although the latter form of prior art infrared radar systems does provide target range, velocity and reflectivity characteristics suitable for use in terrain avoidance and target acquisition and identification, the performance of such systems are substantially limited since such systems cannot provide both MTI and high resolution ranging operation at the same time.
Accordingly, it is an object of the present invention to provide an improved radar system which simultaneously provides range and moving target identification information.
Another object is to provide an improved radar system providing capability of terrain and obstacle avoidance and in addition, target acquisition and identification.
Yet another object is to provide an improved radar system which simultaneously provides active and passive imaging.
It is a further object to provide an improved radar system having a closed-loop fire control capability.